For weapons of this type the breech casing is rigidly affixed to the gun mount but the barrel, the screw mechanism and so forth are among the recoiling parts which take part in the recoil movements of the weapon. In order to permit recoil movements, the barrel must be displaceably mounted in the breech casing.
The gun barrel must be mounted in the breech casing so that sufficient play is provided in the bearing means to permit dimensional changes caused by the heating of the gun barrel during firing. Such play is not desired for other reasons, however, mainly because it gives rise to an unwanted spread of the shots. Such spread of shots, caused by the gun itself upon firing, has previously been accepted mainly due to the fact that the contribution from the fire control equipment has dominated the spread of the shots. Efforts have therefore mainly been concentrated on the problem how to reduce this contribution from the fire control equipment.
Now that it is possible to determine the position of a target more precisely and that more sophisticated computers are used in the fire control equipment, these conditions are changing, however. Previously the contribution to the distribution of shots caused by the fire control equipment was about 3-5 mrad but today it has been reduced to 1-2 mrad; that is, the same magnitude as the gun firing spread. This means that efforts now also are concentrated on the problem of how to reduce also the gun firing spread.
The gun firing spread is mainly caused by the oscillating movements of the gun barrel and the elevating system which movements are generated through firing. The oscillating movements have several causes and may for instance, depend upon the forces in recoil brakes, recoil forces on the upper carriage and forces generated by the screw mechanism when closing, opening and loading the chamber of the barrel. A study of such oscillating movements has disclosed that the play between the barrel and the breech casing gives a significant contribution to the gun firing spread.
Recently measures have been taken to limit also the gun firing spread. And specific means have been introduced into the weapon system for damping the oscillating movements. Such damping means, however, makes the gun constructions heavier and more expensive and has not specifically reduced the gun firing spread caused by the play between the barrel and the breech casing.